Methods of sensing angular-speed within a sensor (such as, for example, a gyro sensor) include a mechanical method, an optical method and a fluid method. The mechanical method is a method utilizing the precession or a body of revolution. The optical method is a method utilizing changes in optical-reception timing, which are caused by the rotation of an enclosure of a laser beam orbiting inside the enclosure. The liquid method is a method whereby gas for sensing the angular speed in an enclosure is injected to heat rays, and changes in injection amount, which are caused by the rotation of the enclosure, are detected as a heat-ray temperature. On the other hand, demand for an angular-speed sensor for detecting the movement direction of a vehicle in a car-navigation system and the like has risen rapidly in recent years. As such an angular-speed sensor, a vibration-type angular-speed sensor, which is inexpensive and light in comparison with the methods described above, has become predominant. In the vibration-type angular-speed sensor, when an angular speed is applied to a vibrator vibrating in a reference direction determined in advance, a new vibration component based on a Coriolis force is detected in a detection direction perpendicular to the reference direction and an angular speed is output based on the detected vibration component.
Systems for controlling a vehicle by using an angular-speed sensor, a vehicle stability control system and a 4-wheel steering-angle control system are commonly known. The vehicle stability control system of a vehicle is a system for keeping the vehicle in a normal state by detecting transversal skid of the vehicle and controlling the brake and torque of the vehicle in an optimal manner. On the other hand, the 4-wheel steering-angle control system is a system for controlling the steering angle of the rear or front wheels of the vehicle. The conventional systems of this type detect an abnormal state of the vehicle as an angular-speed signal generated by an angular-speed sensor. Enhancement of the reliability of the detection of this angular-speed signal is demanded. An example of the abnormal state of the vehicle is the transversal skid cited above.
In relation to the problem described above, JP-2001-153659A and Japanese Patent No. 2,504,233 disclose the following technology. When a drive amplitude of a vibrator employed in a vibration-type angular-speed sensor goes beyond a prescribed range, an abnormality may be generated at a zero point of the output of the angular-speed sensor or in the sensitivity of the sensor. Thus, the drive amplitude is detected to determine whether or not the amplitude is within the prescribed range. In addition, when a vehicle receives a large shock from hitting a curb or the like, the shock is also propagated to an angular-speed sensor mounted on the vehicle. In this case, a signal of a processing circuit in the angular-speed sensor is saturated because of the shock, so that the signal may no longer represent an angular speed (or a yaw rate), which is an original quantity to be detected. In order to solve this problem, the signal output by the angular-speed sensor is examined to determine whether or not the signal exceeds a predetermined level. When the signal output by the angular-speed sensor exceeds the predetermined level, an abnormality is determined to have occurred in the control system of the vehicle. To put it concretely, the drive amplitude of the vibrator is detected by means of a piezoelectric element, being converted into a voltage in a charge-voltage conversion process. The voltage is then rectified into a DC signal, which is used as an amplitude monitor signal. Then, the level of the amplitude monitor signal is examined to determine whether or not the level is within a predetermined range. A level of the amplitude monitor signal beyond the predetermined range indicates an abnormality.
In recent years, however, miniaturization of the vibrator is demanded. To meet this demand, a semiconductor-type vibrator having an electrostatic capacitor as detection means serving as a substitute for the piezoelectric device is gradually being used as a replacement of the conventional vibrator. In this case, however, attachment of infinitesimal foreign particles, which did not raise a problem in the prior system, also has an effect on the sensitivity and zero point of the angular-speed sensor. Consequently, only detection of an abnormality of the vibration amplitude of the vibrator and detection of an abnormality caused by a disturbance and a shock are no longer sufficient.